1. Field of the Invention
The present invention relates to a field effect transistor and a method of manufacturing the same and particularly to a field effect transistor with a gate electrode having a short length and a method of manufacturing the same.
2. Description of the Background Art
Recently, as semiconductor integrated circuit devices are significantly highly integrated, submicrofabrication of elements is rapidly improved. In particular, for a dynamic random access memory (DRAM) as a semiconductor memory device, the integration degree of a memory is increased as its storage capacity is increased from 64 megabits to 256 megabits and furthermore to 1 gigabit. A field effect transistor as an active element which configures such a highly integrated memory must have a submicrofabricated structure.
It is well known that as the length of a gate electrode of a field effect transistor is decreased, drop of the threshold voltage of the field effect transistor, so-called short channel effect, is found. However, when the length of a gate electrode is as extremely short as no more than 0.5 .mu.m, the amount of boron doping is increased to enhance punchthrough resistance, and consequently reverse short-channel effect, increase of threshold voltage, is found simultaneously with short-channel effect.
FIG. 15 is a partial cross sectional view of a structure in cross section of a field effect transistor immediately after impurity ion injection for forming a source/drain region. As shown in FIG. 15, a silicon substrate 1 has a boron doped region 60 formed for controlling the threshold voltage of the field effect transistor. A gate electrode 3 is formed on silicon substrate 1 with a gate oxide film 2 disposed therebetween. A sidewall oxide film 4 is formed at a sidewall of gate electrode 3. The field effect transistor has LDD structure and includes a pair of a lightly doped source/drain regions 51 and a pair of highly doped source/drain regions 52. In the vicinity of the source/drain region immediately after the impurity ion injection, lattice defect, such as interstitial atoms and dislocation loops formed due to the impurity ion injection, is found.
When heat treatment is performed on silicon substrate 1 in the above condition, coupled diffusion of boron (B) atoms contained in boron doped region 60 and the lattice defect is caused. Ultimately, boron concentration peak regions 161, 162 and 163 are formed, as shown in FIG. 16. Boron concentration peak regions 161 and 162 exist in silicon substrate 1 at a certain depth. Boron concentration peak region 163 exists under gate electrode 3 at a surface region of silicon substrate 1. Thus, boron concentration is high at a surface (an interface) of silicon substrate 1 closer to an end of gate electrode 3.
For a long length of an gate electrode, a length d of boron concentration peak region 163 existing at a surface of silicon substrate 1 is relatively short with respect to a length L of the gate electrode and thus reverse short-channel effect is not found. When length L of the gate electrode is decreased, however, the amount of boron doping is increased to enhance punchthrough resistance. This causes length d of boron concentration peak region 163 to be relatively increased with respect to length L of the gate electrode and thus reverse short-channel effect appears.
FIG. 17 shows short-channel effect. As shown in FIG. 17, as gate length is reduced, threshold voltage Vth suddenly drops.
FIG. 18 illustrates reverse short-channel effect found when the gate length is decreased and the amount of boron doping is increased accordingly to enhance punchthrough resistance, as described above. As shown in FIG. 18, as the gate length is decreased in the area representing the relatively long gate length, threshold voltage Vth is increased and the so-called reverse short-channel effect is caused. As the gate length is further reduced, the boron concentration is decreased at the surface of silicon substrate 1 at the center of gate electrode 3, as shown in FIG. 16. This readily causes punchthrough and short-channel effect becomes more significant, as shown in FIG. 18.
In particular, a field effect transistor applied for a DRAM of 1 gigabit has a gate length of approximately 0.15 .mu.m and thus a relatively high ratio of length d of boron concentration peak region 163 to gate length L is obtained. Thus, reverse short channel effect and short channel effect such as those described above become significant, good transistor characteristics cannot be obtained and the field effect transistor will not operate properly.
As shown in FIG. 18, when reverse short-channel effect is found in the area representing the relatively long gate length, a significant short-channel effect is found in the area representing the relatively short gate length. In other words, the amount in reduction of threshold voltage with respect to change of gate length is more increased. When the dependence of threshold voltage on gate length is thus increased, characteristics of the field effect transistor is significantly changed depending on slight variations of process precision.